1. Field of the Invention
The present invention relates to a gauging head for checking linear dimensions of workpieces, including a casing, a gauging arm movable with respect to the casing, a feeler fixed to an end of the gauging arm for contacting the workpiece to be checked, a transducer with at least two elements for providing a signal responsive to the position of the gauging arm and mechanical zero-setting means having movable stop means adapted to be arranged in a first position for defining a determined mutual position of the two transducer elements and in a second position for permitting the mutual displacements of the gauging arm and of at least one element of the transducer.
2. Description of the Prior Art
It is well known that the gauges for checking linear dimensions of mechanical parts may feature very high accuracy and repeatability (e.g. in the micron range), but have normally rather narrow measuring ranges, for example range of some millimeters. The width of the measuring range is basically determined by the transducer. One of the types of transducers more frequently employed is the linear voltage differential transformer (LVDT) transducer, which includes a first element with electric coils and a second element with a core movable with respect to the coils.
In order to utilize the gauges for checking workpieces having dimensions differing by an amount larger than the width of the measuring range of the transducer, the gauges are normally provided with mechanical zero-setting devices.
U.S. Pat. No. 3,863,350 discloses an electronic gauge having a zero-setting device including a replaceable calibrated reference member that permits displacements of the gauging head--having a single feeler--to different positions. Each position of the gauging head defines a relevant zero-setting determined by the calibrated reference member employed.
This technical solution, which permits quick and simple zero-setting operations, may only be used under particular circumstances and requires the availability of several calibrated reference members.
Other known mechanical zero-setting devices permit suitable displacements of one or more elements of gauging heads (e.g. adjustments of the feeler; of an element coupled to the gauging arm and supporting the feeler; of an element of the transducer) thus allowing, in substance, a translation of the measuring range. Gauging heads embodying these features are described in U.S. application Ser. No. 453,108.
It is easily understood that on the one hand it is desirable to have broad measuring ranges in which the gauge performances have the necessary features of accuracy, repeatability and linearity of response and, on the other hand, it is important that the zero-setting device or devices allow zero-setting operations with adjustments of considerable lengths and which are safe, quick and very simple in order that even a poorly skilled operator can carry out the operations with minimum loss of time. Moreover, the possibility of performing the zero-setting operations without any need of auxiliary tools and devices may be important.
These requirements are particularly important for in-process gauging heads for grinders; these gauging heads can check the dimensions (normally internal and/or external diameters, thickness, axial dimensions) of workpieces during grinding operations. These gauging heads also require suitable characteristics with regard to sturdiness, resistance to impacts, minimum sensitivity to vibrations etc.
U.S. Pat. No. 3,122,838 discloses a gauging head for bench checks including a movable gauging arm comprised of two elements connected by a friction screw. The mutual angular position of the two elements can be adjusted by acting on the screw, thus carrying out a rough zero-setting. Moreover, if the element carrying the feeler is subjected to a rather violent accidental impact, this element may rotate with respect to the other element, thus preventing breakages of the rather delicate components of the gauging head.
A technical solution substantially identical to the preceding one is used in the in-process gauging head for checking workpieces during grinding operations described in U.S. Pat. No. 4,238,886. This gauging head, for checking external diameters, comprises two movable gauging arms carrying at an end relevant feelers and at the other end the two elements of a position transducer. Each gauging arm is comprised of two sections coupled by a frictional device. The zero-setting operation is carried out by locking the two sections of the gauging arms which carry the two transducer elements--in such a way that these elements have a determined mutual position--, by acting on the other two sections for displacing the feelers into contact with the workpiece to be checked and by unlocking again the first two sections.
Of course, the frictional devices must offer a rather low resistance, for permitting manual displacements of the two sections of the gauging arms in order that the feelers contact the workpiece, and at the same time a resistance sufficiently high to prevent sliding during the gauging operation.
The two gauging heads described in U.S. Pat. Nos. 3,122,838 and 4,238,886 have the drawback that impacts against the feelers--or the sections of the gauging arms which carry the feelers--or vibrations and other stresses, can cause small sliding displacements of the frictional devices, or in any way sliding displacements not easily perceivable, thus producing loss of the zero-setting condition, which may pass unnoticed to the operator. In this case the operator does not intervene and continues relying on the measurements provided by the gauging head, which as a matter of fact are wrong. If the gauging head signal is utilized for controlling the operation cycle of a grinding machine, this may bring about the obtainment of workpieces having final dimensions out of tolerance.